Signal Slot Qt Thread
While the purpose of threads is to allow code to run in parallel, there are times where threads must stop and wait for other threads. For example, if two threads try to write to the same variable simultaneously, the result is undefined. The principle of forcing threads to wait for one another is called mutual exclusion. It is a common technique for protecting shared resources such as data.
The recommended way of working with threads in Qt has changed since the documentation was written. It is usually better not to add signals, let alone slots, to QThread. Instead, create a QObject (derived) instance, and call moveToThread on it to move it to the thread. Put your signals and slots in this worker object instead.
- Signals & Slots, Signals and slots are made possible by Qt's meta-object system. To one signal, the slots will be executed one after the other, in the order they have valueChanged, and it has a slot which other objects can send signals to. The context object provides information about in which thread the receiver should be executed.
- Some times you see a signal is emitted in sender thread but connected slot doesn't called (in other words it doesn't receive signal), you have asked about it and finaly got that the connection type Qt::DirectConnection would fix it, so the problem found and everything is ok.
Qt provides low-level primitives as well as high-level mechanisms for synchronizing threads.
Low-Level Synchronization Primitives
QMutex is the basic class for enforcing mutual exclusion. A thread locks a mutex in order to gain access to a shared resource. If a second thread tries to lock the mutex while it is already locked, the second thread will be put to sleep until the first thread completes its task and unlocks the mutex.
QReadWriteLock is similar to QMutex, except that it distinguishes between 'read' and 'write' access. When a piece of data is not being written to, it is safe for multiple threads to read from it simultaneously. A QMutex forces multiple readers to take turns to read shared data, but a QReadWriteLock allows simultaneous reading, thus improving parallelism.
QSemaphore is a generalization of QMutex that protects a certain number of identical resources. In contrast, a QMutex protects exactly one resource. The Semaphores Example shows a typical application of semaphores: synchronizing access to a circular buffer between a producer and a consumer.
QWaitCondition synchronizes threads not by enforcing mutual exclusion but by providing a condition variable. While the other primitives make threads wait until a resource is unlocked, QWaitCondition makes threads wait until a particular condition has been met. To allow the waiting threads to proceed, call wakeOne() to wake one randomly selected thread or wakeAll() to wake them all simultaneously. The Wait Conditions Example shows how to solve the producer-consumer problem using QWaitCondition instead of QSemaphore.
Note: Qt's synchronization classes rely on the use of properly aligned pointers. For instance, you cannot use packed classes with MSVC.
These synchronization classes can be used to make a method thread safe. However, doing so incurs a performance penalty, which is why most Qt methods are not made thread safe.
Risks
If a thread locks a resource but does not unlock it, the application may freeze because the resource will become permanently unavailable to other threads. This can happen, for example, if an exception is thrown and forces the current function to return without releasing its lock.
Another similar scenario is a deadlock. For example, suppose that thread A is waiting for thread B to unlock a resource. If thread B is also waiting for thread A to unlock a different resource, then both threads will end up waiting forever, so the application will freeze.
Convenience classes
QMutexLocker, QReadLocker and QWriteLocker are convenience classes that make it easier to use QMutex and QReadWriteLock. They lock a resource when they are constructed, and automatically unlock it when they are destroyed. They are designed to simplify code that use QMutex and QReadWriteLock, thus reducing the chances that a resource becomes permanently locked by accident.
High-Level Event Queues
Qt's event system is very useful for inter-thread communication. Every thread may have its own event loop. To call a slot (or any invokable method) in another thread, place that call in the target thread's event loop. This lets the target thread finish its current task before the slot starts running, while the original thread continues running in parallel.
To place an invocation in an event loop, make a queued signal-slot connection. Whenever the signal is emitted, its arguments will be recorded by the event system. The thread that the signal receiver lives in will then run the slot. Alternatively, call QMetaObject::invokeMethod() to achieve the same effect without signals. In both cases, a queued connection must be used because a direct connection bypasses the event system and runs the method immediately in the current thread.
There is no risk of deadlocks when using the event system for thread synchronization, unlike using low-level primitives. However, the event system does not enforce mutual exclusion. If invokable methods access shared data, they must still be protected with low-level primitives.
Having said that, Qt's event system, along with implicitly shared data structures, offers an alternative to traditional thread locking. If signals and slots are used exclusively and no variables are shared between threads, a multithreaded program can do without low-level primitives altogether.
See also QThread::exec() and Threads and QObjects.
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Introduction
Remember old X-Windows call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offers a new event handling system: signal-slot connections. Imagine an alarm clock. When alarm is ringing, a signal is being sent (emit). And you're handling it in a slot.
- Every QObject class may have as many signals and slots as you want
- You can emit signals only from within that class, where the signal is located
- You can connect signal with another signal (make chains of signals);
- Every signal and slot can have unlimited count of connections with other.
- ATTENTION! You can't set default value in slot attributes e.g. void mySlot(int i = 0);
Connection
Qt Signal Slot Thread Performance
You can connect signal with this template:
QObject::connect (
);
You have to wrap const char * signal and const char * method into SIGNAL() and SLOT() macros.
Qt Signal Slot Thread Context
And you also can disconnect signal-slot:
QObject::disconnect (
);
Deeper
Widgets emit signals when events occur. For example, a button will emit a clicked signal when it is clicked. A developer can choose to connect to a signal by creating a function (a slot) and calling the connect() function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.
For example, if a Quit button's clicked() signal is connected to the application's quit() slot, a user's click on Quit makes the application terminate. In code, this is written as
Qt Signal Slots Across Threads
connect(button, SIGNAL (clicked()), qApp, SLOT (quit()));
Connections can be added or removed at any time during the execution of a Qt application, they can be set up so that they are executed when a signal is emitted or queued for later execution, and they can be made between objects in different threads.
The signals and slots mechanism is implemented in standard C++. The implementation uses the C++ preprocessor and moc, the Meta Object Compiler, included with Qt. Code generation is performed automatically by Qt's build system. Developers never have to edit or even look at the generated code.
In addition to handling signals and slots, the Meta Object Compiler supports Qt's translation mechanism, its property system, and its extended runtime type information. It also makes runtime introspection of C++ programs possible in a way that works on all supported platforms.
To make moc compile the meta object classes don't forget to add the Q_OBJECT macro to your class.
